Tibetan rtswa ‘grass’

In this paper, Guillaume Jacques proposes that the Old Tibetan semi-vowel –w– as part of a word onset is secondary, and that it has its origin in words ending in -u followed by -ba: he supposes the evolution Cu-ba > Cuwa > Cwa, for instance zwa ‘nettle’ < zu-ba, rwa ‘horn’ < ru-ba, grwa ‘corner’ < gru-ba. Another example is ‘grass’, OT rtswa, which must then come from an earlier rtsu-ba. Tibetan rtswa is compared to Chinese 草 *[tsʰ]ˤuʔ > tshawX > cǎo ‘grass, plants’ by Matisoff (here, p. 177) under a reconstruction PST *r-tswa-n, as part of a list of mostly spurious comparisons. In the case of ‘grass’, Matisoff is lucky, but only Jacques’s proposal makes sense of the phonology of this comparison, since OT -a does not otherwise correspond to OC -u.